An increasingly useful, and therefore popular class of ultrasound diagnostic imaging system employs a so-called mechanical sector scanner head. These systems, designed for compactness and easy manipulation, feature a scanning mechanism which is placed in close contact to the skin of the subject by means of coupling gel and oscillates to-and-fro to generate an expanding sector shaped image field in the patient. Thus, for discrete radially extending scan lines in the sector, a transducer fires a sonic pulse into the patient, and an acoustic echo train is returned to and sensed by the transducer. Scanning is achieved in a variety of ways, for example by employing mirrors which oscillate through an angle which establishes the size of the sector. An example of such a system is disclosed and claimed in copending U.S. application Ser. No. 178,482 (UNI-11) of C. Hottinger, entitled "Ultrasound Imaging System Employing Real Time Mechanical Sector Scanner", assigned to the assignee hereof.
It is an object of the present invention to provide oscillatory motive power, and the requisite control mechanisms therefor to establish substantially constant speed oscillatory motion for a scanning component, such as a mirror, in mechanical sector scanner ultrasound systems. Oscillating the mirror at a constant velocity maximizes the number of pulse/echoes that can be transmitted in one field and thus maximizes the number of fields per second displayed on the video monitor.
It is an associated object to provide such apparatus which is of relatively minimum size, without compromise to controllability or reliability.
At least two types of motor/servo systems seem appropriate to meet the objects of the principles of the present invention. Brushless DC motors are quite compact, possess considerable reliability, and eliminate noise and fatigue problems associated with commutating brushes. Brushless DC motors have conventionally been deemed quite difficult to control, however, and largely for this reason have not been extensively utilized in mechanical sector scanner designs. Indeed, traditional approaches to control of brushless DC motors have not only been functionally marginal, but furthermore utilize control approaches which invoke size limitations which the brushless DC motors are introduced to avoid. For example, Hall effect devices and photo transistors have been utilized to determine the peak torque points for the brushless motors, but both approaches have been found wanting due to functional shortcomings and to excessive bulk for the compactness requirements of mechanical sector scanners. Further, should the number of poles in the motor be changed, magnets for the Hall effect sensors would have to be completely redesigned, or the photo transistors employed in the optical control approach would have to be realigned. Moreover, utilization of three phase brushless motors entails considerable difficulty in the detection of overspeed, and the corresponding utilization of the motor to act as a decelerating brake.
In short, although on casual observation brushless DC motors appear excellent candidates for ultrasound mechanical sector scanner head applications, extensive controllability problems have led most designers instead to utilize brush type motors. As previously stated, although these motors are quite readily controlled for the contemplated applications, they entail the severe disadvantages of brush wear (and the need for periodic replacement thereof), and the generation of excessive noise due to the mechanical characteristics of the brushes.
It is an object of the present invention to provide motive and control apparatus for ultrasound mechanical sector scan heads. It is an allied object to provide compact, convenient servo control apparatus which provides stable, reliable control, and which allows the motor to be utilized as a decelerating brake in overspeed situations.
It is a further object of the principles of the present invention to utilize brushless DC motors but to avoid control rationale based on the likes of Hall effect systems, or optical/photo transistor designs.